[ACG48-P02] Improvement in runoff reproducibility of a land surface model and its implications for the energy-water balances on land
Keywords:Runoff, Water table depth, Land surface model, land energy-water balances
We found that the diagnosed groundwater level that was used for calculation of surface runoff (Dunne flow) and base runoff tended to be too deep. Therefore, we tried a simple method to forcibly raise groundwater levels to examine its effect in this study. Specifically, we changed the condition for saturation judgment. In the base experiment (EP.0), a soil layer was judged saturated when soil moisture averaged over a grid cell was almost equal to saturation. In the sensitivity experiments, the condition was relaxed to about 75% (EP.1), 50% (EP.2), 25% (EP.3), and 0% (EP.4) of saturation. A series of numerical experiments was conducted in the Chao Phraya River basin (CP) in Thailand as a case study. The meteorological data from 1981 to 2004 at a horizontal resolution of 5 arc-minutes (i.e., approximately 9 km) (Kotsuki et al., 2013) was used.
The calculated monthly and daily river flow in EP.2 agree well with observation at the Bhumibol Dam (BB) on the Ping River in the northwestern part of CP. The groundwater level in the catchment of BB in EP.2 were much shallower (seasonal range was 1-3 m) than that in EP.0 and EP.1 (5-6 m). The seasonal changes in the vertical profiles of monthly mean soil moisture, and the monthly Bowen ratio were also improved.
The overall behavior of the model was dramatically improved by changing the condition of saturation judgment. The improvements in the Bowen ratio and soil moisture suggests that those impacts will be amplified when the land surface model is couled to the other components of climate models. In the results above, the streamflow and Bowen ratio agreed well with the measurements when the saturation threshold was at about 50% of saturation (EP.2). The good agreement in the experiment with the relaxed threshold implies that there are processes that yield large runoff at a sub-grid scale due to inhomogeneity of land surface, such as macro pore, landscape, land use, etc. An optimum saturation threshold is expected to exist, that simulates such local runoff.